The present invention relates to nuclear magnetic resonance (NMR) apparatus and, more particularly, to a novel radio-frequency (RF) coil of the "birdcage" form, having optimized signal-to-noise ratio (SNR) particularly when receiving RF signals in a NMR imaging or spectroscopy environment.
It is now well known to image and obtain chemical shift spectra from certain nuclei, such as .sup.1 H, .sup.31 P and the like, to determine the internal distribution and chemical form of those nuclei in a specimen, using NMR. In particular, NMR imaging of a particular nuclear species in the human body has proven to be medically and scientifically significant. It is now also well known that substantial increases in SNR can be observed by increasing the static magnetic field B.sub.O in which the specimen-to-imaged is immersed. It has been found that attempts to reduce the size of a volume picture element (voxel) by trading off this SNR advantage, result in relatively little reduction of the linear voxel extent, since the voxel linear dimension is apparently proportional to the cube root of the signal-to-noise ratio. However, a SNR increase can be utilized to shorten the data acquisition time, with acquisition time reductions proportional to the square of the SNR increase being realizable. Accordingly, it is highly desirable to increase, to the greatest extent possible, the signal-to-noise ratio in a NMR RF volume coil, and particularly in the form of RF volume coil known as a "birdcage" coil, which is one having a pair of conductive loop elements spaced along a common longitudinal axis, with each of the loop elements having a plurality of series-connected capacitance elements spaced along the loop peripheries, and with a like plurality of axial conductive elements electrically interconnecting the conductive loop elements at points between adjacent ones of the serially-connected capacitive elements. such as are described and claimed in U.S. Pat. Nos. 4,680,548 and 4,692,705, respectively issued July 14, 1987 and Sept. 8, 1987, both assigned to the assignee of the present application and both incorporated herein in their entireties by reference.